GVP said in 2007: “The rarely visited Lastarria has not erupted in historical time, but has displayed strong fumarolic activity for at least 67 years. This is the first Bulletin report ever issued on this volcano; it presents new images of the steaming edifice…”

Not that the volcano wasn’t known as such – persistent fumarolic activity has been reported from the northwestern flanks since the earliest records were made. However, even the most recent eruptions appear to have predated the Spanish colonists: no records of large or small eruptions have ever been found. Yet, in 1900, Dr. L. Darapsky said “Lastarria volcano… is the only one in the district which shows signs of volcanic activity, exhaling sulphur and water steams“. Mine workers had seen steam rising from it since at least 1940, and Casertano reported solfataric and vapour activity in the 1960s. But it was only in 1986 that the Chilean volcanologist J. A. Naranjo presented a first detailed study of the volcano. Today Lastarria is monitored, like the other active volcanoes in Chile, by SERNAGEOMIN.

That there is a volcano just sitting and steaming for hundreds of years without people having any knowledge of former eruptions is one puzzling situation in itself. But this was getting more enigmatic when scientists learned about a steady inflation below the area, which was even two-tiered just below Lastarria volcano. Since then, quite a number of studies have been done by international scientists.

On the map of hazards for the Antofagasta region, Volcán Lastarria is marked as moderately dangerous with a quite large area of risk for lahars and pyroclastic flows.

Lastarria volcanic complex lies in the in the Antofagasta Region, at the western edge of the sparsely populated Puna Plateau, and on the border between Chile and Argentina. With its large deposits of sulfur, its stark beauty and stunning nearby volcanic landscapes, it is one of Earth’s few areas still unspoilt by tourism and infrastructure. There are very few settlements in this arid region within 150 km around the volcano. Access is difficult, just an unpaved road leads to Lastarria from the former Catalina railway station 120 kilometres west of it. The volcano is named in honour of José V. Lastarria (1817-1888) who was a notable Chilean writer, revolutionary and politician. Occasionally I have seen the Lastarria named Volcán Azufre (Sulfur).

LASTARRIA + AZUFRE = “LAZUFRE”

Volcán Lastarria is located in the southern part of the Central Andes, where the continental crust is estimated to be ~60-70 km thick. It also lies on the southern edge of the Altiplano-Puna Volcanic Complex, which had made headlines a few years ago by the discovery of an underlying large magma body (agimarc wrote a post about it in 2017).

The purple colored complex on the left is the “Lazufre”, with Lastarria on its northern tip. (img. P. Grosse et al., 2017)

Even though Lastarria is a stratovolcano-dome complex itself, it also makes up the northern part of the Azufre Volcanic Complex. This includes Cordón del Azufre, the next volcano immediately south of Lastarria. The entity is commonly abbreviated as “Lazufre”. Together it is a 30 km long chain of overlapping Quaternary volcanic centers, aligned in a NE-SW direction, between Lastarria and Cerro Bayo volcanoes. “Lazufre” has recently become an area of scientific interest in itself because it turned out to be one of the largest deforming volcanic systems on Earth (see below).

The location is some 300 km east of the subduction trench. The basement is made up of andesite-dacite volcanic rocks, i.e. ignimbrites, lava flows and lava domes of Miocene to Pleistocene age eruptions. Here a major crustal lineament known as the Archibarca fault intersects the main volcanic arc. The intersection between this lineament and the arc may act as a weakness zone that promotes the ascent of magma.

THE VOLCANO

Lastarria volcanic complex. On the lower left the massive lava flows of the Negriales field can be seen; to the north lie extensive PF deposits, and a debris flow from flank failure went to the right. (Google Earth)

Lastarria is a polygenetic complex, andesitic to dacitic in composition. The volcanic history shows a northward migration of craters along a 10 km-long ridge. SERNAGEOMIN gives its last major eruption as 2460±60 years ago, calculated from pyroclastic flow deposits. Nevertheless, at least one younger pyroclastic deposit has been identified. If smaller events did happen in historical times nobody has written them down.

Lastarria’s present alert level is “Green” – an active volcano with stable behavior. From SERNAGEOMIN’s report of 10/2018*:
– 1153 seismic events associated with fluid dynamics inside the volcano
– 1043 of them were long period quakes
– 110 of them were tornillos.
– It is thought that most of this activity was generated by the hydrothermal system.
– Degassing from the fumarolic fields reached 300 m above the crater.

*Since SERNAGEOMIN renders their report PDFs un-translate-able I can only hope I’ve got these details right…

Naranjo identified three distinct structures that form the Lastarria complex:

Lastarria proper
(L. sensu stricto) in the North, which is the main edifice and presently active. It is built by five NW-SE aligned nested craters and has four fumarolic fields with active vents and extensive fumarolic deposits. The modern composite cone consists of lava flows, scoria and pyroclastic flow deposits, and one dome. At least three different generations of lava flows (upper, middle, and lower) can be distinguished.

All other flanks of the volcano are mainly covered with pyroclastic deposits and are difficult to access – due to land-mine fields close to the Chilean-Argentinian border.

Recent pyroclastic flows form an extensive apron on the northern flanks up to the shore of Laguna de la Azufrera, more than 8 km to the N. One of the most impressive recent events was a flank failure on the SE side: it produced a debris avalanche which had such a speed and energy that it completely climbed and overran a 125 m high older cone in its path.

A major slope failure on the SE flank of Lastarria caused a debris flow that completely climbed and over-rode the northern half of an old scoria cone of 125 m hight. (GEarth)

The avalanche was made up of light and loose porous scoria and pumice which made it very mobile and fast. Perhaps it had also entrained cushions of air at its flow front and so become even faster. This high velocity would provide the energy to climb up as it did.

Negriales lava field
located in the SW portion of the complex. It was formed by several impressive (up to 10 km long) lava flows with flow ridges and levees. It erupted several 100k years ago from a single fissure in the SE and is now plugged by a lava dome. This was the source of the largest single eruption (~14 km³) in the region of Lastarria, and it is much greater than any single flow from the main edifice.

South Spur
(Espolón Sur) is a strato cone in the SSW and joined by a ridge to the main cone at 5500 m altitude. This is the ancestral volcano in the complex. It has two craters and several andesitic lava flows.

Lastarria Volcanic Complex is an active volcanic system which displays all the colorful and smelly features of fumarolic emissions and their abundant deposits as well as extensive hydrothermal alteration zones. Strong and persistent fumarolic activity is found on the SW and E side of the youngest crater and on a fracture system down the NW flank of the main volcanic edifice. The fumarolic products are mainly native sulphur crusts in bright yellow, orange, reddish to almost black colors, but also other minerals of high sulfurization (realgar, orpiment and cinnabar).

One of the most peculiar sights of the Lastarria volcano is its molten sulfur flows and pools. Naranjo writes in 1985: “On Earth, fumarolic sulphur is very common in almost all active volcanoes, but sulphur flows are rare, having been reported only on Siretoko–Iôsan volcano in Japan, on Sierra Negra and Azufre volcanoes in the Galápagos Islands, and on Mauna Loa in Hawaii. Of these, the Siretoko–Iôsan example is the only natural molten sulphur flow to have been observed in the process of formation; the others being rather poorly exposed.” Here, the flows are rivulets of liquid sulfur flowing max. ~350 m down, like little lava flows, even sometimes forming a ropy texture. They result from melting of fumarolic sulphur deposits in a hotter environment, due to renewed magmatic activity.

Liquid sulfur pool, Lastarria volcano. Screenshot from a Twitter video by F. Aguilera, check out his other videos and photos: @FelipeVolcanes)

CHANGES IN THE VOLCANIC WORKINGS

Inflation

Scientists observed that the entire “Lazufre” volcanic area is currently undergoing active deformation. They found it to be one of the largest volcanic systems on Earth, comparable in size to Yellowstone or Long Valley. A roughly elliptical area of ~ 50×38 km is being pushed up at a rate of ~3 cm/yr (others say even more). This phenomenon has been monitored since 1997 and it is assumed that the deformation has started between 1996 and 1998. The depth of the source for the uplift has been estimated between 9 and 17 km.

There have been five or six theories as to what causes this inflation. All but one were about processes that occur in an over-pressured magma reservoir. One of them considered pulses of heat from the magmatic system to an overlying hydrothermal aquifer as the cause (without changes in magma volume). Updated geodetic modeling (2017) describes the inflation as a large (20×30 km), steadily growing sill-like magma body intruding into elastic crust below “Lazufre”.

In addition to the large deep inflation, a second, much smaller deformation area was detected only ca. 1 km deep and directly below the S part of Lastarria volcano. Since 2003 this area was inflating up to 1.5 cm/yr. Unfortunately, no more deformation studies have been conducted since 2008, so, currently (2017) it is not known if this shallow source has continued to inﬂate. It may be assumed, though, as both the large and the small inflation developed the same way during 2003–2005, suggesting that the two sources could be closely related.

Differences in composition of Lastarria’s volcanic gases pose new challenges to volcanologists. Recent measurements indicate that a notable change in Lastarria’s magmatic-hydrothermal system likely occurred between 2009 and 2012. The 2014 measured gas composition contained relatively higher proportions of H2O and SO2 and relatively low proportions of CO2, which would be consistent with degassing of a shallow magma body. Scientists interpret these changes as either
– due to magma ascent, or
– degassing of a stalled magma body, or
– changes in the hydrothermal system,
but they deem the options 1 and 2 as the most likely.

Magma storage

As usually, the more one learns the more questions start popping up. Using the technology of noise tomography, Spica et al. found in 2015 the presence of hydrothermal and magmatic reservoirs below the “Lazufre” volcanic area. It now became necessary to also find out about Lastarria’s magma plumbing and hydrothermal systems.

In 2017, A. Stechern et al. developed a model of the magma plumbing system beneath Lastarria volcano. They used the geo-chemical properties of historical lava samples to prove the existence of a complex magma system.

Earlier studies with seismological and MT tomography had shown that the magma reservoir is not precisely below Lastarria’s summit but a few km southward.

Investigations and research are still going on around Volcán Lastarria. With most monitored volcanoes, there have been eruptions in the not-too-distant past; volcanologists could use personal observations, measurements and analyses of the volcanic products to draw conclusions for possible future behaviour. Not so with a volcano that has never been seen erupting, its products being old, eroded and even changed chemically. Everything has to be found out from scratch and compared with other, more active neighbors. It takes years to provide an acceptable model for the possible behaviour of one individual volcano. So, it will be very exciting to see the upcoming results – especially for such a volcano that had behaved “as if” (it wanted to erupt) for quite a time.

Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment – corrections are always welcome!